1. Field of the Invention
The present invention relates generally to photomultiplier detectors and, more particularly, to the reduction of hysteresis in such detectors. The invention is particularly useful in applications, such as spectrophotometry, requiring linear sensitive detectors of low light levels.
2. Description of the Prior Art
Photomultipliers have found widespread use in applications requiring sensitive detectors of low light levels. In a photomultiplier detector light flux impinging on a photo-emissive cathode is converted into electrons released from the cathode. The electrons are directed at and cascade down a series of dynodes at each of which the number of electrons increases by a process called secondary emission. The multiplied number of electrons is ultimately collected at an anode and is measured as an electrical current. A photomultiplier is a very sensitive light detector since the gain or ratio of anode current to cathode current may be as high as 10.sup.8 or more.
The dynode or electron multiplying section of a photomultiplier detector comprises a plurality of dynodes in series. Insulating spacers support the dynodes and insulate each from one another and from other elements of the detector. A predetermined interdynode voltage is applied between each dynode and the next to cause the electrons to cascade from one dynode to the next toward the anode. The gain of the photomultiplier depends, in part, on the number of dynodes, the inter-dynode voltage between dynodes, and the electrostatic fields that channel the electrons from one dynode to the next.
A serious problem encountered in photomultiplier detectors is output current variation or hysteresis causing corresponding nonlinearity in the gain characteristic of the detector. A principal cause of such hysteresis is the unstable nature and erratic behavior of the electrostatic fields within the detector housing. In this regard, electrons scattered within the photomultiplier housing collect on the insulating spacers supporting the dynodes and on other elements or portions of the housing. The resulting electrostatic charge may continue to increase for periods from seconds to minutes and as it changes will effect similar changes in electrostatic focusing of the electrons from one dynode to the next. Moreover, the rate and equilibrium level of this charge buildup is a function of the light level striking the photocathode and the dynode voltage. These factors combine to cause the undesired variation or hysteresis in the output current and corresponding nonlinearity in the detector gain characteristic.
The problem of photomultiplier hysteresis has long plagued the field of spectrophotometry. Basically, many spectrophotometers employ a photomultiplier to measure the extent to which light is transmitted through or absorbed by a sample material of unknown characteristics. This measurement is compared with a corresponding transmittance or absorbance measurement of a reference material of known characteristics. Spectrophotometers for this purpose can be broadly categorized as double beam or single beam in design and operation. In a double beam spectrophotometer the sample material and the reference material are measured in rapid sequence in separate optical paths, the beams of light passing through the sample and through the reference are combined into a common beam, and this common beam is passed to a single photomultiplier detector. The detector output is demodulated to derive a signal indicating the difference between the sample and the reference measurements and hence indicating the desired sample measurement. Fortunately, in a double-beam spectrophotometer, the time constant for hysteresis of the detector gain is long relative to the time lapse between sample and reference measurements. Consequently, gain is equal for sample and reference readings and linearity is preserved. However, the price paid for this performance is the inherent design complexity and resulting higher costs inherent in the construction of a double-beam system.
In a single-beam spectrophotometer, on the other hand, the sample and the reference are measured at different times in the same optical path. As a result, a single beam instrument is particularly vulnerable to detector hysteresis since hysteresis alone will cause the detector output to change with time and light level introducing non-linearity. Because of the hysteresis problem, the art has not been able to capitalize on the relative design simplicity, lower noise level, and lower cost of a single-beam system, and many users have found it necessary to settle for the more costly and complex double-beam design.
Several solutions to photomultiplier detector hysteresis have been proposed in the past. In a first solution the exposed surface area of insulation is reduced within the detector housing to minimize the surface area available for the buildup of electrostatic charge. However, the insulation itself can only be reduced a finite amount before all elements of the detector are shorted out rendering the detector inoperative. In a second solution the insulating spacers are located as far as possible from the paths of electron travel between the dynodes. However, this expedient is limited in application since it requires fabrication of large and expensive photomultiplier housings. In addition to the above two solutions, it is known to employ "insulating" spacers exhibiting some degree of electrical conductivity. This prevents electrostatic charge buildup by conducting charge away through the spacers themselves. This arrangement has only limited hysteresis reduction capability in view of the fact that a leakage path exists between dynodes.
In another solution, exemplified by U.S. Pat. No. 3,943,458, one intermediate dynode is connected as the anode, and the remaining dynodes between it and the original anode are at least partially inactivated by connecting each and the original anode to a point in the dynode bias resistor chain between the intermediate dynode serving as the anode and the preceding active dynode. As a result it is possible for the inactivated dynodes to compete with the intermediate dynode (i.e. the new anode) for electrons emanating from the preceding active dynode. At high anode current levels especially, secondary electrons will leave the intermediate dynode/anode and collect on the inactive dynodes causing a nonlinearity between anode current and light intensity.
In view of the complexity and cost in the above approaches for reducing photomultiplier hysteresis, there are only a few relatively inexpensive so-called "low-hysteresis" photomultiplier detectors available. One of these is the type R928HA photomultiplier manufactured and sold by Hamamatsu TV Company, Ltd. In a publication of the manufacturer entitled "Hamamatsu Photomultiplier Tubes" dated February 1979 the subject of photomultiplier hysteresis in photometric applications is discussed in general at page 8 where it is stated that "Hysteresis may be a problem in applications such as photometry. To eliminate hysteresis, the ceramics on the top and bottom of the electrodes of Hamamatsu side-on types are coated with chromium and maintained at the cathode potential." The existence of a cathode potential adjacent to the (chargeable) insulator repulses electrons and they are therefore less inclined to collect on these insulators. The repulsion field increases with dynode voltage and at high voltages becomes quite effective in reducing hysteresis.
Unfortunately, efforts to incorporate the foreging R928HA photomultiplier into a single beam spectrophotometer were unsuccessful. It was found that an acceptable level of hysteresis could not be sustained across the full range of dynode voltage required to accommodate the different light levels to be measured. In this respect, to accommodate a maximum range of light levels, the photomultiplier gain is increased for low light levels and is decreased for higher light levels. The preferred method of adjusting the detector gain is by adjusting dynode voltage. However it was found that at low dynode voltages hysteresis was excessive. On the other hand, at high dynode voltages the hysteresis was acceptably low, but the detector gain was excessive. Excessive gain resulted in high anode current and other difficulties such as fatigue and non-linearity introduced by an unequal voltage developed between dynodes.
As a result, there is a need for a photomultiplier detector adopted for spectrophotometric systems exhibiting the desirable aspects of the prior detector, such as low cost, without the attendant disadvantages. The present invention meets these needs.